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copied!<p>It’s possible to load a symbol file in gdb with the <code>add-symbol-file</code> command. The hardest part is to produce this symbol file.</p> <p>With the help of libMachObjC (which is part of <a href="https://github.com/nygard/class-dump" rel="noreferrer">class-dump</a>), it’s very easy to dump all addresses and their corresponding Objective-C methods. I have written a small tool, <a href="https://github.com/0xced/class-dump/tree/objc-symbols" rel="noreferrer">objc-symbols</a> which does exactly this.</p> <p>Let’s use Calendar.app as an example. If you try to list the symbols with the <code>nm</code> tool, you will notice that the Calendar app has been stripped:</p> <pre><code>$ nm -U /Applications/Calendar.app/Contents/MacOS/Calendar 0000000100000000 T __mh_execute_header 0000000005614542 - 00 0000 OPT radr://5614542 </code></pre> <p>But with <code>objc-symbols</code> you can easily retrieve the addresses of all the missing Objective-C methods:</p> <pre><code>$ objc-symbols /Applications/Calendar.app 00000001000c774c +[CALCanvasAttributedText textWithPosition:size:text:] 00000001000c8936 -[CALCanvasAttributedText createTextureIfNeeded] 00000001000c8886 -[CALCanvasAttributedText bounds] 00000001000c883b -[CALCanvasAttributedText updateBezierRepresentation] ... 00000001000309eb -[CALApplication applicationDidFinishLaunching:] ... </code></pre> <p>Then, with <a href="https://github.com/0xced/SymTabCreator/" rel="noreferrer">SymTabCreator</a> you can create a symbol file, which is just actually an empty dylib with all the symbols.</p> <p>Using <code>objc-symbols</code> and <code>SymTabCreator</code> together is straightforward:</p> <pre><code>$ objc-symbols /Applications/Calendar.app | SymTabCreator -o Calendar.stabs </code></pre> <p>You can check that <code>Calendar.stabs</code> contains all the symbols:</p> <pre><code>$ nm Calendar.stabs 000000010014a58b T +[APLCALSource printingCachedTextSize] 000000010013e7c5 T +[APLColorSource alternateGenerator] 000000010013e780 T +[APLColorSource defaultColorSource] 000000010013e7bd T +[APLColorSource defaultGenerator] 000000010011eb12 T +[APLConstraint constraintOfClass:withProperties:] ... 00000001000309eb T -[CALApplication applicationDidFinishLaunching:] ... </code></pre> <p>Now let’s see what happens in gdb:</p> <pre><code>$ gdb --silent /Applications/Calendar.app Reading symbols for shared libraries ................................. done </code></pre> <p>Without the symbol file:</p> <pre><code>(gdb) b -[CALApplication applicationDidFinishLaunching:] Function "-[CALApplication applicationDidFinishLaunching:]" not defined. Make breakpoint pending on future shared library load? (y or [n]) n </code></pre> <p>And after loading the symbol file:</p> <pre><code>(gdb) add-symbol-file Calendar.stabs add symbol table from file "Calendar.stabs"? (y or n) y Reading symbols from /Users/0xced/Calendar.stabs...done. (gdb) b -[CALApplication applicationDidFinishLaunching:] Breakpoint 1 at 0x1000309f2 </code></pre> <p>You will notice that the breakpoint address does not exactly match the symbol address (0x1000309f2 vs 0x1000309eb, 7 bytes of difference), this is because gdb automatically recognizes the function prologue and sets the breakpoint just after.</p> <p><hr></p> <h2>GDB script</h2> <p>You can use this GDB script to automate this, given that the stripped executable is the current target.</p> <p>Add the script from below to your <code>.gdbinit</code>, target the stripped executable and run the command <code>objc_symbols</code> in gdb:</p> <pre><code>$ gdb test ... (gdb) b +[TestClass randomNum] Function "+[TestClass randomNum]" not defined. (gdb) objc_symbols (gdb) b +[TestClass randomNum] Breakpoint 1 at 0x100000ee1 (gdb) ^D </code></pre> <hr> <pre><code>define objc_symbols shell rm -f /tmp/gdb-objc_symbols set logging redirect on set logging file /tmp/gdb-objc_symbols set logging on info target set logging off shell target="$(head -1 /tmp/gdb-objc_symbols | head -1 | awk -F '"' '{ print $2 }')"; objc-symbols "$target" | SymTabCreator -o /tmp/gdb-symtab set logging on add-symbol-file /tmp/gdb-symtab set logging off end </code></pre>

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